There are gyros for detecting angular velocity of a rotating object. Among them, an optical gyro is characterized by high accuracy. In the optical gyro, angular velocity is detected using the difference in frequency between two laser lights that travel in the opposite directions to each other on a ring-shaped path. An optical gyro employing a rare gas laser has been proposed as such an optical gyro (see, for example, JP11(1999)-351881A). Such an optical gyro allows interference fringes to be formed by extracting laser lights that circulate on the same path in the opposite directions to each other. A general configuration of such an optical gyro is shown in FIG. 16. In the optical gyro shown in FIG. 16, the interference fringes are expressed by the following Formula (1):
                    I        =                              I            0                    ⁡                      [                          1              +                              cos                ⁡                                  (                                                                                    2                        ⁢                                                                                                  ⁢                        π                        ⁢                                                                                                  ⁢                        ɛ                        ⁢                                                                                                  ⁢                        χ                                            λ                                        +                                          2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      t                                        +                    ϕ                                    )                                                      ]                                              (        1        )            
In the above formula, Io denotes the light intensity of laser light, while λ indicates the wavelength of the laser light. Furthermore, ∈ denotes the angle shown in FIG. 16, while χ indicates a coordinate in the X direction shown in FIG. 16. Moreover, Δω denotes the difference in frequency between a clockwise mode and an anticlockwise mode, which occurs when the gyro rotates. In addition, t indicates time. The difference Δω is proportional to the angular velocity Ω of the rotation of the gyro. In other words, Δω=4AΩ/(Lλ), where A denotes the area surrounded by a ring shape, while L indicates an optical path length. Furthermore, φ denotes the initial phase difference between two laser lights. In this gyro, its rotational speed and rotation direction are detected through the detection of the moving speed and direction of interference fringes. The optical gyro employing a rare gas laser, however, has problems in that it requires high voltage for its drive, which results in great power consumption, and it is large and is susceptible to heat.
A gyro that employs a semiconductor ring laser provided with a ring-shaped (triangular ring, quadrilateral ring, etc.) waveguide has been proposed as a gyro that solves such problems (see, for example, JP2000-230831A). The semiconductor laser that is used in this gyro includes a ring-shaped waveguide with an approximately constant width. In the gyro, two laser lights that circulate on the ring-shaped waveguide in the opposite directions to each other are extracted and the interference fringes formed thereby are detected. However, laser lights confined using a narrow waveguide spread widely when going out of the waveguide. It therefore is difficult to detect the interference fringes practically with high accuracy. Hence, common gyros in which a semiconductor laser is employed are: a gyro that detects the beat frequency corresponding to the difference in frequency between two laser lights based on the change in voltage between two electrodes of the semiconductor laser (see, for example, JP4(1992)-174317A); and a gyro that detects the beat frequency using evanescent light that has leaked out of the end face of a resonator (see, for example, JP2000-121367A).
However, the gyro that detects the beat frequency requires an extra device for detecting the rotation direction.